Sea water : Sea water is thewater salt of the seas and oceans of the Earth. It is said to be "salty" because it contains dissolved substances, the mineral salts, made up of ions, mainly halide ions like the chloride ion and alkaline ions like the sodium ion. There are 30 to 40 g of dissolved salts for 1 kg of seawater. Salt water opposes theeau douce, which contains less than 1 g of dissolved salts per kilogram. The density of seawater at the surface is approximately 1,025 g / ml, 2,5% higher than that ofeau douce (1 g / ml) because of the mass of the salt and the electrostriction.
Volume: The oceans and seas occupy an estimated volume of 1 million km338, which represents 3% of the water reserves present on the surface of the Earth. This volume does not include groundwater (aquifers), about 96,5% of which is salty to varying degrees.
Origin of salt from sea water: The first scientific theories on the origin of sea salt date from Edmond Halley who proposed in 1715 that salt and other minerals are brought there by rivers: the flow of water water on the surface (rainwater) and in rivers dissolves ions by dissolving rocks. Fresh underground and surface waters are therefore very slightly "salty" because of these ions; these are carried to the ocean, where they remain while evaporation causes their concentration. Halley notes that the few lakes that do not flow into an ocean (such as the Dead Sea and the Caspian Sea, see Endorheism) generally have very high salinity, which he calls "continental weathering" (continental weathering).
Halley's theory is largely correct. In addition to the phenomenon described above, sodium was stripped from the ocean floor during the initial formation of the oceans. The presence of the other dominant ion in salt, the chloride ion, comes from the "degassing" of hydrogen chloride (in the form of hydrochloric acid) as well as other gases from the interior of the Earth, via volcanoes and hydrothermal vents. Sodium and chloride ions then became the major constituents of sea salt.
The average salinity of the oceans has been stable for several billion years, most likely thanks to a tectonic and chemical process that removes as much salt as it arrives from rivers. Chloride and sodium ions are thus removed by evaporite deposits (“saline rocks”), “gresification” (the deposition of salts in pore water) and chemical reactions with basalt in the seabed5. Since the creation of the oceans, sodium is no longer torn from the ocean floor, but captured in the layers of sediment covering the seabed. Other theories claim that plate tectonics drag some of the salt below land masses, where it slowly rises to the surface.